Preheat oven to 180 C (350 F). Butter (or line with parchment paper) a 20cm x 20cm (8in ×8in) square pan.
In a large bowl, stir together the oats, flour, cinnamon, salt, baking powder, and sweetener. Stir in the butter, then firmly press 2/3 of the dough into the bottom of the pan.
Press the berries on top of the dough in the pan, then sprinkle the reserved dough on top. Press down firmly again.
Bake for 30-35 minutes. Let cool, then refrigerate at least 1 hour before cutting.